Issue #2053 - Part 1: Performance should be an EventTarget

Some sites (e.g. Bitbucket) are now calling `performance.addEventListener` and expect that Performance is an EventTarget.

Based on https://bugzilla.mozilla.org/show_bug.cgi?id=1432522

dom/webidl/Performance.webidl

@@ -14,7 +14,7 @@ typedef double DOMHighResTimeStamp;
 typedef sequence <PerformanceEntry> PerformanceEntryList;
 
 [Exposed=(Window,Worker)]
-interface Performance {
+interface Performance : EventTarget {
   [DependsOn=DeviceState, Affects=Nothing]
   DOMHighResTimeStamp now();